Compact electric grinding machine

ABSTRACT

The present invention particularly relates to a hand-held sanding machine with an outer housing ( 1 ), a tool shaft ( 2 ) and a brushless electric drive motor. In the present invention, the rotor of the drive motor is fastened to the tool shaft ( 2 ) of the sanding machine, and the stator ( 6 ) is positioned in the outer housing ( 1 ). The present invention also relates to a control method for an electric sanding machine.

TECHNICAL FIELD

The present invention relates to an compact electric hand-held sandingmachine with an outer housing and a tool shaft.

PRIOR ART

Electric sanding machines of the same type are previously known from,for example, U.S. Pat. No. 0,245,182. Here, the intention has been tomake a relatively compact and low sanding machine by using a brushlessmotor and making the proportion between the motor diameter and the motorheight great. The drawback of this solution is that the motor diameterunavoidably becomes large and therefore also difficult to be grippedwith one hand. Further, since the diameter is large, it becomesdisadvantageous to make a hermetic motor with cooling only on theoutside. This is very disadvantageous because the air in which thesanding machine is most often used is filled by dust particles that maybe both electrically conductive and grinding by nature.

Since electric sanding machines have previously been so large and heavy,it has been necessary to have special sanding machines in, for example,wall grinding. Machines of this type are previously known from, forinstance, U.S. Pat. No. 5,239,783 or EP0727281. In these patents, asanding machine for walls has been made by moving the motor to the farend of the arm and by, for example, using a cable for transmitting powerto the grinding head. In this way, balance has been achieved for themachine, but this also makes the machine expensive and difficult tomanufacture.

Within the EU and many other markets, there are regulations on how muchinterference may be generated to the network. Within the EU, standardEN61000-3-2 with amendment A14 is applied. If a switched control unit ismade in the simplest way possible by rectifying the mains voltageaccording to FIG. 5 and subsequently having so large a capacitor thatthe following control can continuously take current until the followingpulse comes, very high harmonic components are obtained which interferewith the electric network.

There are two conventional ways of solving this problem: A passive wayby filtering the current and voltage with inductances and capacitors,and an active way. The passive way requires space as well as a greatvolume and weight. The active way functions in such a way that thevoltage is first switched with the known “step-up” topology according toFIG. 6 in such a way that the relation between the input current and theinput voltage corresponds to a resistive load. The output voltage isalways higher than the top value of the input voltage. The drawback withthe active way is that the current goes through an extra inductance L1and is, in addition, switched one more time, because the powercorrection is always followed by a switched control unit.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to alleviate above-mentioneddisadvantages. The sanding machine according to the invention ischaracterized in that it has an electric drive motor that is brushlessand without a shaft of its own, mounted in such a way that the rotor isfastened to the tool shaft and the stator is positioned in the outerhousing. A sanding machine constructed in this way has a compactstructure allowing the sanding machine to be gripped ergonomically withone hand. The compact structure enables a manner of use and a device inthe form of an arm that can be fastened to the sanding machine for acomfortable two-hand grip and an extended range of operation for themachine. At the same time, the invention enables a hermetic structure inwhich the cooling air passes only on the outside of the stator and whichis thus very insensitive to impurities in the cooling air. Since thesanding machine also has a low profile, the control of the grindingproperties of the machine is good.

The motor type used in the invention is what is called BLDC (BrushlessDirect Current) motor. Due to the strong magnetic field of the new NdFeBmagnets, the motor has high power per volume and high efficiency. Thanksto these features, it has been possible to make the motor sufficientlysmall to enable this invention. An advantageous solution is to use aslotless version of the BLDC motor. The slotless motor has smaller ironlosses and a more advantageous price because the iron core of laminationstacks has a simpler form, and the winding is simpler to carry out.

The tool shaft has a eccentrically positioned tool holder and the rotorof the drive motor without own shaft is fastened to the tool shaft, andstator is positioned in the outer housing. The tool shaft is arranged toextend within the rotor of the drive motor and therefore replaces it'sown shaft.

The prior art tools have a tool shaft that is without eccentricity andbalance weights. The eccentricity is made with an external part that isfastened to the motor shaft. The reason for making the eccentricity withan external part is that then the rotor can be fastened to the shaft inconventional way and the bearings can be assembled from both ends. Thissolution makes the sander much less compact.

The cooling air is generated by a blower that is mounted on the toolshaft and can advantageously be integrated in the same verticaldirection as the balance weights of the tool shaft. The same cooling airthat cools the motor first cools the control unit.

Since, thanks to the present invention, the sanding machine is muchlighter and more compact than known electric sanding machines, specialsanding machines intended for wall grinding have become completelyunnecessary. Previously, it has been necessary to make the grinding headlighter by moving the motor to the other end of the arm but with theconsequence that transmission with a cable or shafts is needed. Thepresent sanding machine can be fastened to the end of an arm in such away that it is freely mobile in one or more flexible directions. Sincethe sanding machine is so light, it is still as easy to handle asspecial wall sanding machines having complicated and expensivetransmission. If dust extraction is needed, it is advantageous to leadthe extraction to a hollow arm.

Control of the motor is carried out electronically to be able to varythe speed of rotation. The control unit is made in such a way that thespeed of rotation is kept on a given level irrespective of the load ofthe machine. The control unit may preferably be positioned in connectionwith the sanding machine. A preferable solution is to use sensorlesscontrol, i.e. control without a sensing device to determine the positionof the rotor in the electronic commutation. The sensorless controlusually utilizes the voltage generated in the phase that is notconducting to determine the position of the rotor.

The position of the rotor in the electric commutation can also bedetermined on the basis of the currents generated in the differentphases or the relation between current and voltage in the phases.

When the control is sensorless, the motor is more compact because thesensors, most often Hall sensors, make the motor considerably longer.

According to the new preferred solution for a switched control unit, themotor is dimensioned in such a way that the nominal voltage of the motoris lower than the top value of the rectified mains voltage. When currentis consumed during that part of the cycle when the voltage is higherthan the nominal voltage of the motor and no current is consumed whenthe voltage is lower than the nominal voltage of the motor, differentdegrees of power correction are obtained, depending on how much lowerthe nominal voltage is. If the time during which the current correspondsto an optimal load in relation to the whole cycle is sufficiently long,the harmonic components generated back to the electric network will bewithin the allowed values. When mains voltage of 230 V is rectified, atop value of 325 V is obtained. If the nominal voltage of the motor is,for example, 200 V, there is a current flow approximately 60% of thetime. The current is generated in such a way that no current flows whenthe rectified mains voltage is equal to the nominal voltage, and itincreases linearly in such a way that the current is 10 A when thevoltage is 325 V. This gives an effective power of approximately 1,100W. The third harmonic current component is thus 2.4 A, which is withinthe allowed limit for a portable hand tool. The other harmoniccomponents also have allowed values. Since the windings of the motorform a coil with self-inductance L1, the switched control unit can alsobe preferably made without external inductances.

BRIEF DESCRIPTION OF THE FIGURES

The invention is described in more detail in the following withreference to the attached drawings, in which

FIG. 1 shows a top view of the sanding machine;

FIG. 2 shows a side view of the sanding machine;

FIG. 3 shows a cross-section along line A-A;

FIG. 4 shows a cross-section along line B-B;

FIG. 5 shows an electricity drawing of prior art control;

FIG. 6 shows prior art power correction;

FIG. 7 shows a first embodiment of new motor control; and

FIG. 8 shows a second embodiment of the motor control.

PREFERRED EMBODIMENTS

The sanding machine shown in FIGS. 1 to 4 is formed of a housing 1enclosing all parts of the motor. The motor is formed of a stator 6,including a casing with cooling fins 12 and a rotor 7. These parts areintegrated with the parts keeping a tool shaft 2, a bearing housing atboth ends 4, 11 and a bearing 10 in place, in such a way that the rotor7 is fastened to the tool shaft 2. The casing and cooling fins of thestator 6 are shaped in such a way that an air slot is generated which islimited by the casing, the housing of the sanding machine and thecooling fins. The grinding disc 3 is fastened freely rotationally to thetool shaft 2 via an eccentric bearing 8. The blower 9, which is fastenedto the tool shaft 2 preferably at the same height as the balanceweights, sucks in air through the hole 14. The air cools the controlunit 15 and then the motor via the cooling fins 12. The air is blown outthrough the hole 5. The shroud 16 collects the grinding dust that issucked out through the grinding disc 3 and further out through theextraction pipe 17. The switch 13 is in connection with the control unitand attends to the switching on and switching off ergonomically. A softpart 18 around the casing makes the machine grip-friendly. In anotherembodiment, the disc is not freely rotating, but the disc is rotatingwith or without eccentric movement through a connection to the toolshaft 2.

The rotor is prevented to rotate in respect to the shaft with a woodruffkey where the corresponding slot is shown in the rotor in picture 4. Therotor can also be prevented to rotate with other kind of keys or withsplines.

The balance weights that are integrated in the tool shaft are so largethat the bearing 10 (lower part) has to be mounted before the rotor isfastened to the tool shaft.

To enhance the compactness the bearings 10 and the bearing housings 4,11 are partly or fully inside the stator 6 or the windings.

In another embodiment, the dust extraction also attends to the coolingof the motor in such a way that part of the air is sucked via the motorand the cooling fins, and in this way the motor cools off without aseparate blower.

Functioning of the power correction of the control unit in a firstembodiment is described in FIG. 7. The mains voltage is rectified andthe following capacitor C2 is so small that the voltage follows therectified voltage. The motor is dimensioned in such a way that thenominal voltage of the motor is so much lower than the top value of therectified mains voltage in relation to the required power that powercorrection is obtained when current is consumed during that part of thecycle when the voltage is higher than the nominal voltage of the motorand no current is consumed when the voltage is lower than the nominalvoltage of the motor. The control unit utilizes the well-known“step-down” topology in such a way that the relation between current andvoltage is optimized so that smallest harmonic components possible aregenerated, and thus also the best possible power correction is achievedin that part of the cycle in which the voltage is higher than thenominal voltage of the motor. If the voltage is lower than the nominalvoltage, no power is taken to the motor. If the time when the currentcorresponds to the optimal load in relation to the whole cycle issufficiently long in relation to the required power, the harmoniccomponents generated back to the electric network will be within allowedvalues. If the self-inductance L1 of the motor is sufficiently great,the control unit can preferably be made without external inductances.The motor in FIG. 7 has been simplified in such a way that only oneswitch SW1 is shown. In practice, electronically commutated 3-phasecontrol is directly carried out for the motor.

If the power correction obtained is not sufficient, the function can befurther improved according to the embodiment in FIG. 8. Here, anexternal inductance L1 and an extra switch according to “step-up”topology have been incorporated to carry out power correction alsoduring the time when the voltage is lower than the nominal voltage ofthe motor. The connection is still preferable because the current andthe voltage are lower than in a case where the power correction shouldbe carried out during the whole cycle. Above all, the value at theexternal inductance L1 may be lower because the voltage is lower whenthe switching is carried out.

The above description and the related figures are only intended toillustrate a present solution for the structure of a sanding machine.Thus, the solution is not confined merely to the above or the embodimentdescribed in the attached claims, but a plurality of variations oralternative embodiments are feasible within the idea described in theattached claims.

1-18. (canceled)
 19. A hand-held sanding machine comprising: an outerhousing, a tool shaft and a brushless electric drive motor including arotor, a stator is positioned in the outer housing wherein the toolshaft has an eccentrically positioned tool holder said tool shaft beingcollinearly aligned with said rotor, and the rotor of the drive motorwithout its own shaft is removably fastened to the tool and wherein saidtool shaft extends within the drive motor as part of the rotor.
 20. Asanding machine according to claim 19, wherein the axis of the rotor andthe axis of the tool shaft are spaced apart parallel axes.
 21. A sandingmachine according to claim 19, wherein the grinding disc is mounted onthe tool shaft eccentrically freely rotationally.
 22. A sanding machineaccording to claim 21, wherein the grinding disc is with or withoutreduction gear mounted on the tool shaft in such a way that a rotatingmovement is generated.
 23. A sanding machine according to claim 19,wherein the control unit of the motor is arranged in such a way that thespeed of rotation is constant irrespective of the load.
 24. A hand-heldsanding machine with an outer housing, a tool shaft and a brushlesselectric drive motor, a stator is positioned in the outer housing,wherein the tool shaft has a eccentrically positioned tool holder, andthe rotor of the drive motor without its own shaft is removably fastenedto the tool shaft, which extends within the rotor of the drive motor;wherein the control unit of the motor is arranged in such a way that thespeed of rotation is constant irrespective of the load and, wherein thecontrol unit is a sensorless control unit.
 25. A sanding machineaccording to claim 24, wherein the sensorless control unit is arrangedto determine the position of the rotor in the electronic commutation bythe voltage generated in the phase that is not conducting.
 26. A sandingmachine according to claim 24, wherein the sensorless control unit isarranged to determine the position in the electronic commutation by thecurrents generated in the different phases or the relation betweencurrent and voltage in the phases.
 27. A hand-held sanding machine withan outer housing, a tool shaft and a brushless electric drive motor, astator is positioned in the outer housing, wherein the tool shaft has aeccentrically positioned tool holder, and the rotor of the drive motorwithout its own shaft is removably fastened to the tool shaft, whichextends within the rotor of the drive motor; further including a controlunit, where the mains voltage is rectified and the following capacitor(C2) is dimensioned so small that in use the voltage follows therectified mains voltage and thus current is consumed from the networkduring the time when the voltage is loaded, wherein the motor isdimensioned in such a way that the nominal voltage of the motor is to beso much lower than the top value of the rectified mains voltage inrelation to the required power that power correction is obtained whencurrent is consumed during that part of the cycle when the voltage ishigher than the nominal voltage of the motor and no current is consumedwhen the voltage is lower than the nominal voltage of the motor.
 28. Asanding machine according to claim 27, wherein the relation betweencurrent and voltage is optimized in that part of the cycle in which thevoltage is higher than the nominal voltage of the motor so that smallestharmonic component possible is generated, and thus also the bestpossible power correction is obtained.
 29. A sanding machine accordingto claim 27, wherein the switched power aggregate uses only the motor'sown inductance (L1) as the inductive component in the switching.
 30. Asanding machine according to claim 19, wherein the motor is cooled by ablower which is mounted on the tool shaft.
 31. A sanding machineaccording to claim 19, wherein the cooling air cools the stator of themotor by flowing through the slot generated between the inside of theouter housing and the outside of the stator.
 32. A sanding machineaccording to claim 19, wherein the stator is shaped in such a way thatit is, at the same time, the housing of the sanding machine.
 33. Asanding machine according to claim 32, wherein the stator has built-incooling channels.
 34. A sanding machine according to claim 19, whereinthe structure of the stator, the bearing housings and bearings, is ahermetic structure in which the cooling air passes only on the outsideof the motor.
 35. A sanding machine according to claim 28, wherein powercorrection is carried out also during that time when the rectifiedvoltage is lower than the nominal voltage of the motor.